net: l2: Clear arp cache when disable interface.

When connect to diffrent router with the same gateway ip address,
need to clear arp cache when disable interface,
or it will use the wrong gateway mac address.
Call net_arp_clear_cache function replace to set arp_table 0.

Change-Id: Ib403a0c0030832ba48824db4d2d3fcb8add63d16
Signed-off-by: june li <junelizh@foxmail.com>